The present invention relates to oil and gas well proppants and, more particularly, to sintered proppants made from ingredients which can be uncalcined, partially calcined or fully calcined, a method of making such proppants, and to a method of maintaining a fracture in a subterranean formation in a propped condition by utilizing such proppants.
Oil and natural gas are produced from wells having porous and permeable subterranean formations. The porosity of the formation permits the formation to store oil and gas, and the permeability of the formation permits the oil or gas fluid to move through the formation. Permeability of the formation is essential to permit oil and gas to flow to a location where it can be pumped from the well. Sometimes the permeability of the formation holding the gas or oil is insufficient for economic recovery of oil and gas. In other cases, during operation of the well, the permeability of the formation drops to the extent that further recovery becomes uneconomical. In such cases, it is necessary to fracture the formation and prop the fracture in an open condition by means of a proppant material or propping agent. Such fracturing is usually accomplished by hydraulic pressure, and the proppant material or propping agent is a particulate material, such as sand, glass beads or ceramic pellets, which are carried into the fracture by means of a fluid, such as oil or water.
Spherical pellets of uniform size are believed to be the most effective proppant body shape due to maximized permeability. For this reason, assuming other properties to be equal, spherical or essentially spherical proppant bodies, such as rounded sand grains, metallic shot, glass beads and tabular alumina, are preferred. Examples of prior art proppants and their use are found in U.S. Pat. Nos. 2,950,247, McGuire, et al.; 3,026,938, Huitt, et al.; 3,126,056, Harrell; 3,497,008, Graham, et al.; 3,976,138, Colpoys, et al.; and 4,068,718, Cooke, et al.
There has been an evolution in the development of manmade proppants since the 1970xe2x80x2s. The early manmade proppants were produced from high alumina based feedstocks such as bauxite, a material commonly used to produce high alumina bodies. Bauxite was a popular feedstock because it was readily available, many of the early producers had a good working knowledge of the material in conjunction with prior work in abrasives and refractories, and the cost was consistent with initial demands of the enhanced oil and gas recovery market. The early proppants had the advantage of high strength when compared with the traditional proppant, sand, used in hydraulic fracturing, a basic method of enhanced oil and gas recovery. The high strength of the alumina can be directly related to the high specific gravity and therefore bodies fabricated from these materials have high densities.
The hydraulic fracturing process requires that the proppant be suspended in a fluid and pumped under pressure into the well. To maintain the suspension of the proppant the viscosity of the fracturing fluid must be capable of keeping the proppant suspended. For deep wells, the high specific gravity and associated high viscosity required to adequately fracture a well, may be acceptable when maximum proppant strength is required. Not all oil and gas wells are of sufficient depth to require proppants with high strength. It is especially true of shallow depth wells that have been successfully enhanced by fracturing using sand as the propping agent.
By the late 1970xe2x80x2s it became evident that there was a need for a proppant with lower density, lower cost, and the presumably lower strength. During the next two decades proppants became available with lower specific gravities. The density or specific gravity of these proppants was reduced by replacing the alumina content having a theoretical density of 3.95 grams per cubic centimeter with silica having a theoretical density of 2.5 grams per cubic centimeter. The original bauxite based proppants of the early 1970xe2x80x2s contained  greater than 80% alumina (Cooke). Subsequent generations of proppants contained an alumina content of  greater than 70% (Fitzgibbons), 40% to 60% (Lunghofer), and later 30% to  less than 40% (Rumpf, Fitzgibbons).
The other major element typically contained in proppants is silica. As mentioned above, the replacement of the alumina with the silica resulted in a proportionate drop in the density of the proppant material and a decrease in strength in most cases. It is believed that the proppant""s strength was associated with the crystalline corundum phase, mullite phase, and/or silica-cristobalite phase (Rumpf). Many prior inventors expressed a desire to maintain the silica contained in the raw material and/or sintered proppants in either the amorphous phase or cristobalite phase. In addition, many prior inventors recommended that the quartz phase of silica was to be specifically avoided or limited in proppant compositions. The quartz phase is known to undergo an inversion at 573xc2x0 C. from xcex1 to xcex2 form during which a thermal expansion occurs upon heating and is reversed during cooling. Inversions of this type are often associated with cracking or inducing a stress in a quartz containing body subject to thermal treatment above the inversion temperature of 573xc2x0 C.
As a result, it is desirable to develop a proppant having a composition with a highly reduced bauxite component. In addition, it is desirable that any such proppant material maintain relatively high permeability as well as high end strength so that it is capable of effective use in shallow as well as deep wells.
It is an object of the present invention to provide a proppant composition capable of being used as an oil and gas well proppant that includes a reduced amount of bauxite and/or alumina.
It is another object of the invention to provide a proppant composition that includes quartz as a material component of the composition.
It is still another object of the invention to provide a method for forming the above composition which can employ either a high intensity mixer or a spray fluidizing bed to form spherical pellets.
It is still a further object of the invention to provide a method for forming the proppant composition which sinters the spherical pellets utilizing a rotary kiln, a box kiln, or a fluidized microwave sintering bed.
The subject of this invention is a proppant using as its raw materials; quartz, shale containing quartz, bauxite, talc, and wollastonite. The resultant proppant made from such raw materials may contain as much as 65% quartz, and has yielded sufficient strength to be used in wells to a pressure of 10,000 pounds per square inch. Also, the resultant proppant has an alumina content of less than 25% by weight, and a silica content of greater than 45% by weight.
The quartz and shale representing  greater than 60% of the raw materials are inexpensive and readily available. Quartz is one of the most readily available and least expensive materials in the world. (Ref. Crystalline Silica Primer, U.S. Bureau of Mines, Dept. of the Interior, pp 11,12). The use of uncalcined bauxite, including the weight of the crystal water, represents  less than 33% of the raw material required to produce the proppant and is of a low grade and is considered relatively inexpensive. The alumina content of the bauxite is low and contributes  less than 25% and preferably  less than 20% of the total proppant weight. The talc and wollastonite represent  less than 10% of the raw material and are available on the world market to satisfy the desire to utilize low cost materials. The source of the silica may be from sand, clay (in the form of fine particles of hydrous aluminum silicates) and the bauxite.
The raw materials may contain low amounts of surface moisture that need not be removed. The surface moisture need only be remove if it is excessive or if the moisture removal is required by the preferred crushing and grinding method to prevent agglomeration. The raw material must be ground to an acceptable size, however there is no requirement to eliminate the crystal water by heat treatment of the raw material prior to crushing and grinding. The ground raw materials are blended and formed into spheres and are then thermally processed to increase the sphere""s density and associated strength.
The high level of quartz and reduced amounts of bauxite and/or alumina in the proppant body combined with the ability to maintain the quartz phase and attain a high degree of physical strength after thermal processing at temperatures between 1150xc2x0 C. and 1170xc2x0 C. is considered unique. In addition, it has been found that it is possible to recycle the proppant body through the thermal processing cycle and enhance the strength of the proppant. The ability to recycle the proppant body through the thermal cycle and enhance its strength is considered a significant advantage. The ability to thermally recycle the proppant is not addressed in other patent literature and it is believed that at least some of the prior art proppant bodies do not respond to thermal reprocessing.
This invention differs from prior art in that the primary crystalline phase is a quartz, the secondary phases are hematite and xcex1 alumina. The bauxite is the source of the hematite and the a alumina. A tertiary interstitial phase of complex silicates binds the larger primary and secondary crystals. The addition of talc and wollastonite results in the formation of a magnesium iron silicate (spinel) phase and an anorthite feldspar phase respectively. There is an absence of amorphous glass, mullite, and the cristobalite phase of silica reported in prior art. The mullite and cristobalite are replaced by a quartz, a alumina, and hematite. The spinel and feldspar are interstitial crystalline phases between the primary and secondary phases and acting as a high strength binder. The iron content can be reduced by beneficiation of the bauxite or by the selection of a bauxite with a lower iron content.
In accordance with the present invention, a proppant composition and a method for forming the composition has been developed that forms pellets containing one or more preferably uncalcined or possibly partially or fully calcined ingredients in an alumina-to-silica dry weight basis ratio of from about 1:3 to about 1:2. The proppant composition includes a binder formed of wollastonite and talc that holds the components of the composition together in an unexpected manner to produce a proppant with good permeability, low density and reduced weight which is similar to other low-bauxite content proppants, but having a high end strength, which is similar to high-bauxite content proppants. Silver and Baryte uncalcined bauxite from Greece has been found to be particularly useful although bauxites from other sources may be employed.
The present invention thus provides a gas and oil well proppant comprising a mixture of from about 1% by weight to about 10% by weight talc, from about 1% by weight to about 10% by weight wollastonite, from about 5% by weight to about 33% by weight uncalcined bauxite, preferably from about 30% to about 32% by weight uncalcined bauxite, from about 10% by weight to about 65% by weight quartz, preferably from about 30% to about 32% by weight quartz, and from about 10% by weight to about 65% by weight uncalcined shale, preferably from about 30% to about 32% by weight uncalcined shale wherein the material has an alumina content of less than about 25% by weight, and a silica content of greater than about 45% by weight.
The materials which are particularly adapted for use to form the proppant composition of the present invention include uncalcined or partially calcined clays and uncalcined or partially calcined bauxites, shales and quartz. These materials are blended with the binder formed of wollastonite and talc to produce composite sinterable, spherical pellets. The pellets are subsequently heated to produce sintered, spherical pellets eminently useful as proppants. The composites of the present invention are preferably made from an uncalcined or partially calcined ingredients. However, the composites of the present invention can also be made from calcined ingredients.
The present invention also provides a process for propping fractures in oil and gas wells at depths of 6,000 to 14,000 feet utilizing the present sintered pellets by mixing the pellets with a hydraulic fluid, such as oil or water, and introducing the mixture into a fracture in a subterranean formation. The compaction pressure upon the fracture generally is at least 280 kg/cm2 (4,000 psi) and usually is in the range of from about 350 to about 700 kg/cm2 (5,000 to about 10,000 psi). The present pellets have an average particle size between 0.1 and 2.5 millimeters. It has been found that the present composite pellets containing 50 percent or more by weight quartz and held together by the wollastonite/talc binder have desirable permeability characteristics at pressures up to about 700 kg/cm2 (10,000 psi). The true density of proppant of this invention has been measured at 2.623 gm/ml and 2.632 gm/ml for the 16/20 mesh size and 20/40 mesh sizes respectively. The loose packed bulk density has been measured at 1.51 gm/cc for the 16/20 mesh size proppant of this invention.
The present proppant materials are produced by forming a mixture comprised of dried but uncalcined, or partially calcined bauxite, shale, quartz, wollastonite, and talc. A temporary binder formed from the addition of starch is used to improve pelletizing and increase the green strength of the unsintered pellets. Each of the starting ingredients has an average particle size of less than about 15 microns and, preferably, less than about 10 microns.
In one preferred method, the mixture is produced on an intensive mixer having a rotatable table provided with a rotatable impacting impeller, such as described in U.S. Pat. No. 3,690,622, Brunner. Sufficient water is added to cause essentially spherical ceramic pellets to form within the mixer.
The unsintered pellets may also be formed by a spray-granulation process. In this process, the mixture of all of the ingredients used in forming the pellets is formed into an aqueous feed suspension including the mixture and the temporary binder. The feed suspension is subsequently atomized in a layer of partly dried particles that are fluidized in a stream of heated drying air. Finished particles are then removed from the fluidized layer and separated in order to recover particles having the desired size and shape.